This application claims priority to an application entitled An Apparatus for Fabricating Apodized Fiber Grating filed in the Korean Industrial Property Office on Jul. 2, 1999 and assigned Serial No. 99-26671, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to fiber gratings, and in particular, to an apparatus for fabricating apodized fiber gratings.
2. Description of the Related Art
As the data transmission capacity of a WDM (Wavelength Division Multiplexing) system increases, channel spacing gets narrower. Therefore, there is an increasing need for optical filters that have a narrow bandwidth and excellent adjacent channel isolation characteristics.
Fiber gratings satisfy the requirements of such optical filters, i.e., low loss, low polarization dependence, and high channel selectivity. Further, the cost effectiveness of the fiber gratings makes them popular as optical filters.
When a general fiber grating is fabricated in a conventional method using an excimer laser and a uniform phase mask, the refractive index of the fiber is constant over the length of the grating. In such a fiber, however, a sidelobe occurs and as a result, no apodization is achieved at the fiber grating. This sidelobe can be reduced by apodizing the fiber grating such that the magnitude of a refractive index variation is decreased toward the ends of the fiber grating.
An apodized fiber grating refers to a fiber grating of which the refractive index increases or decreases toward the center or both ends. The apodized fiber grating shows minimized sidelobes in both a short wavelength band and a long wavelength band. Although this apodization is effective in reducing a sidelobe in a longer wavelength band, it has limitations in reducing a sidelobe in a shorter wavelength band due to self-induced chirping of a fiber grating.
The self-induced chirping is attributed to an inconstant average refractive index of the fiber grating. Accordingly, the average refractive index should be made constant with respect to grating length in order to reduce a sidelobe which arises from the self-induced chirping.
Other conventional fiber grating apodizing methods besides the conventional method discussed above include overlap writing, use of a PZT (Piezo Transducer), optical scanning, and use of a spatial filter. The overlap writing method is called an interference method, in which apodization is achieved by writing gratings superimposed on other gratings of different periods and sizes in an optical fiber.
As another conventional apodization method, gratings are written on an optical fiber while a tensile force is applied to the optical fiber using a piezo transducer. During writing the gratings, the optical fiber or a phase mask is vibrated for a desired length in the length direction of the optical fiber by the use of the piezo transducer.
Thirdly, apodized gratings are written by scanning an optical fiber covered with a phase mask with UV (Ultraviolet) light lengthwise with different light intensities at different scanning rates.
A spatial filter operates based on light interference. In this method, the intensity of interference light passed through a diffraction slit exhibits a Gaussian distribution. A spatial filter with a different transmission is disposed before a phase mask along the length direction of an optical fiber and UV light is projected onto the phase mask.
Other examples of fiber gratings and manufacturing methods of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 5,712,715, to Erdogan et al., entitled OPTICAL TRANSMISSION SYSTEM WITH SPATIALLY-VARYING BRAGG REFLECTOR, describes a Bragg grating produced using two collimated non-collinear beams to form an interference pattern.
U.S. Pat. No. 5,717,799, to Robinson, entitled OPTICAL WAVEGUIDE FILTERS, describes a reflection pass-band filter with particular chirp and apodization profiles. The apodized grating is produced by varying the strength of the writing of the grating elements as a function of distance along the fiber.
U.S. Pat. No. 5,912,999, to Brennan III et al., entitled METHOD FOR FABRICATION OF IN-LINE OPTICAL WAVEGUIDE GRATING OF ANY LENGTH, describes a method and apparatus for writing apodized Bragg gratings. In this method, the intensity of the writing beam is varied to control the envelope of the refractive index profile to write an apodized grating.
U.S. Pat. No. 5,953,472, to Boschis et al., entitled METHOD OF AND A DEVICE FOR MAKING BRAGG GRATINGS IN OPTICAL FIBERS OR WAVEGUIDES, describes a method of making gratings in which the fiber is illuminated through a phase mask. A diaphragm is used to give the beam a Gaussian intensity distribution.
U.S. Pat. No. 6,035,083, to Brennan III et al. entitled METHOD FOR WRITING ARBITRARY INDEX PERTURBATIONS IN A WAVEGUIDING STRUCTURE, describes a method in which a waveguide is translated relative to a writing beam, and the writing beam is modulated as a function of time to write the grating. Apodization can be achieved by controlling the amplitude envelope of the writing beam modulation.
U.S. Pat. No. 6,043,497, to Quetel et al., entitled PHOTO-IMPRINTING STAND FOR THE MAKING OF BRAGG GRATINGS, describes a photo-imprinting stand which has a dynamic masking device with a variable surface. The dynamic device may have a rotating shutter with a particular profile, and masks UV rays during the photo-imprinting process.
The above conventional apodized fiber grating fabricating methods have the following problems:
(1) In overlap writing, a device for accurately controlling a length smaller than a grating period is required for appropriate overlapped writing, thereby making it complicated to fabricate fiber gratings;
(2) In the use of a piezo transducer, it is also difficult to control a length smaller than a grating period reliably;
(3) In optical scanning, the optical scanning rate and optical intensity must be controlled appropriately to obtain a desired apodized grating; and
(4) With use of a spatial filter, vibrations must be prevented since gratings are fabricated using interference patterns, and for this purpose an expensive device is required.
Especially, when a fiber grating is fabricated using a phase mask, the phase mask should be fabricated by focused ion beam implantation and wet etching to have an effective profile. A new phase mask is needed at every change in apodization conditions. Therefore, this method is not effective in terms of cost and flexibility.
Despite the advantage of production of gratings with various characteristics, the method of scanning an optical fiber lengthwise with UV light at a controlled light intensity has the distinctive shortcomings of long fabrication time and bad reproducibility.
Consequently, the method using an apodizing phase mask is not effective in terms of cost and flexibility since a phase mask is difficult to fabricate and a new phase mask is needed at every change in apodization conditions. Moreover, the beam scanning method has the disadvantages of difficult fabrication, long fabrication time, and bad reproducibility.
It is therefore an object of the present invention to provide an improved method and apparatus for fabricating an apodized fiber grating.
A further object of the invention is to provide a method and apparatus for fabricating an apodized fiber grating with a short fabrication time.
A yet further object is to provide a method and apparatus for fabricating an apodized grating with high reproducibility.
A still further object of the invention is to provide a method and apparatus for fabricating an apodized grating which are less expensive.
Another object of the invention is to provide an apparatus which does not require expensive vibration damping equipment.
Yet another object of the invention is to provide an apparatus and method which do not require a separate phase mask for different apodization conditions.
Still another object of the present invention to provide an apparatus for fabricating an apodized fiber grating readily using a beam splitter and a screen mask.
Yet another object of the present invention to provide an apparatus for fabricating an apodized fiber grating with a uniform refractive index distribution in the length direction.
To achieve the above objects, in an apodized fiber grating fabricating apparatus according one aspect of the present invention, a UV (Ultraviolet) laser emits a UV laser beam, a beam splitter splits the UV laser beam emitted from the UV laser into two beams, a plurality of mirrors form light paths to concurrently project the split beams onto an optical fiber from two directions by reflecting the split beams, a phase mask passes the reflected beams therethrough in such a way to form gratings in the optical fiber in a predetermined period, a first blocking device is disposed between the phase mask and one of the mirrors, progressively blocks one of the two beams from being projected toward the optical fiber from one direction, and provides apodization to the formed gratings, and a second blocking device, which is mobile and opposite to the first blocking device with respect to the optical fiber, progressively blocks the other beam from being projected toward the optical fiber from another direction and provides apodization to the formed gratings, so that an average refractive index variation is constant across the whole gratings.
In an apodized fiber grating fabricating apparatus according to another aspect of the present invention, a first UV laser emits a first UV laser beam toward an optical fiber from one direction and a second UV laser emits a second UV laser beam toward the optical fiber from an opposite direction. A phase mask forms gratings in the optical fiber in a predetermined period by reinforcement and interference of the first UV laser beam. A first blocking device, disposed between the first UV laser and the phase mask, progressively blocks one of the beams from being projected to the optical fiber and thus provides apodization to the formed gratings, and a second blocking device opposite to the first blocking device with respect to the optical fiber, progressively blocks the other beam from being projected to the optical fiber and provides apodization to the formed gratings, so that an average refractive index variation is constant across the whole gratings.